The present invention relates to a display control system for controlling color graphic data to be displayed by a raster scanning, color graphic display unit.
Generally, no image signals are transmitted during the horizontal scanning period, i.e., part of the vertical blanking period of a television signal. However, teletext systems allow character data and figure data (i.e., digital signals) to be transmitted to a receiver terminal during the horizontal scanning period. These pieces of data are temporarily stored in the image memory provided in the receiver terminal. They are read from the memory and supplied to the raster scanning, color graphic display unit of the receiver terminal.
Various standard image display formats have been proposed for this teletext system. Among them is a format consisting of 248 (horizontal).times.204 (vertical) picture elements. To shorten the time for transmitting the image data of this format and to reduce the price of the terminal, the unit of data to be controlled for coloring and flashing consists of four picture elements, i.e., 4 (horizontal).times.4 (vertical) elements, and does not represent an individual picture element. This unit will be hereinafter called a "functional block."
To control the brightnesses of the 16 picture elements of each functional block, i.e., 4.times.4 dot pattern DP, a 16-bit luminance data signal is required. Further, 4-bit FG (foreground) color data, 4-bit BG (background) color and 4-bit data attribute CC data are allotted to each functional block. The FG color data and the BG color data each consist of 1-bit red data R, 1-bit green data G, 1-bit blue data B and 1-bit intensity-lowering data RI.
To display the image data stored in the image memory, each 4-bit dot pattern DP is scanned in the horizontal direction, thus reading 4 bits from the image memory. For the same purpose, the 4-bit FG/BG color data and 4-bit data attribute CC allotted to pattern DP (i.e., the functional block) are then read from the image memory. The image data thus read out of the memory is displayed by the raster scanning, color graphic display unit.
In most teletext system of this type, image data is processed in units of eight bits and is written in and read from the memory through an 8-bit data bus. Hence, four 8-bit pieces of data, i.e., an 8-bit dot pattern DP, an 8-bit FG color data, an 8-bit BG color data and an 8-bit attribute data CC must be read from the image memory during the period of displaying eight picture elements (8 bits) in the horizontal direction.
The image memory used in most teletext systems is a dynamic random access memory (DRAM). A DRAM has a large capacity, and its cost per bit is low. However, its cycle time is 200 to 260 nsec, and its accessing time is relatively long. Thus, when clock pulses of 5.73 MHz (the pulse interval: approx. 175 nsec), i.e., a frequency 8/5 times that (3.58 MHz) of the color sub carrier, are used to read the image data from the DRAM and then to convert it to serial data, one-clock period of 175 nsec is too short; therefore, 2 clock, periods, i.e., 350 nsec, are necesary. Therefore, in a conventional display control system, the four pieces of 8-bit data are serially converted to serial data and are read from an image memory in an 8-clock period. In this case, all of the 8-bit clock period is used for only the reading of data as shown in FIG. 1. The conventional display control system is, therefore, disadvantageous in that no data can be written into the image memory during the displaying period, inevitably reducing the data-writing efficiency.
This disadvantage can be eliminated by using a static RAM having a short accessing time. Data can be written into the static RAM even during the displaying period by using the cycle steal technique. However, the static RAM is expensive, and it is difficult to design hardware for a high-speed, accurately timed operation.
Also, four pieces of data can be written into the image memory during the displaying period if the address area of the memory is divided into four parallel sub-areas as showing FIG. 2. This method, however, leaves a large part of the memory vacant, and the resultant image memory is larger than necessary.
In some teletext systems, each receiver terminal has two image memories for storing two frame-images and two display control devices for controlling the image memory and reading the two frame-images independently from the memory so that a hybrid display is achieved by combining the two images. Obviously, the receiver terminal is inevitably larger and more expensive than the receiver terminal with only one such display control device.